OPHIDTA. 



403 



side of the anal aperture, and never exhibiting any' outward 

 evidence of their existence beyond the occasional presence of 

 short horny claws or spurs ("calcaria"). In the entire absence, 

 then, or rudimentary condition of the limbs, the Snakes progress 

 by means of the ribs. These bones are always extremely nu- 

 merous (sometimes amounting to more than three hundred 

 pairs), and in the absence of a sternum they are, of course, ex- 

 tremely movable. Their free extremities, in fact, are simply 

 terminated by tapering cartilages, which are attached by mus- 

 cular connections to the abdominal scales or " scuta " of the 

 integument. By means of this arrangement the Serpents are 

 enabled to progress rapidly, walking, so to speak, upon the 

 ends of their ribs : their movements being much facilitated by 

 the extreme mobility of the whole vertebral column, conditioned 

 by the cup-and-ball articulation of the bodies of the vertebrae 

 with one another. 



The body in the Snakes is covered with numerous scales, 

 developed apparently in the lower layer of the epidermis, and 

 covered by a thin, translucent, superficial pellicle, which is peri- 

 odically cast off and renewed. On the head and along the 

 abdomen these scales are larger than over the rest of the body, 

 and they constitute what are known as the " scuta " or shields. 



The only other points in the anatomy of the Ophidia which 

 demand special attention are the structure of the tongue, teeth, 

 and eye. 



The tongue in the Snakes is probably an organ more of 



Fig. 157. A, Diagram of the eye of a Serpent (after Cloquet): a Ball of the eye 

 covered by a conjunctival sac, into which the lachrymal secretion is discharged ; 

 b Optic nerve ; d Antocular membrane, formed by the epidermis ; e e Ring of scales 

 surrounding the eye. B, Head of the common Viper (Pelias berus) after Bell 

 showing the bifid tongue, and the poison-fangs in the upper jaw. 



touch than of taste. It consists of two muscular cylinders, 

 united towards their bases, but free towards their extremities. 

 The bifid organ, thus constituted, can be protruded and 



